In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter and powdered sugar until fully combined.
Add in the flour, black cocoa, vanilla extract, and salt and mix until fully combined and smooth. The dough will feel a bit dry but should come together. Transfer the dough into an airtight container and refrigerate overnight (but a minimum of 2 hours if you can’t wait all night).
Line a baking sheet with parchment paper. On a lightly floured surface, roll out the dough ¼ inch thick and then cut out your cookies.
Place the cookies 1-2 inches apart on your baking sheet (they won’t spread much). Place the baking sheet in the fridge or freezer for a minimum of 20 minutes before baking to allow the butter in the dough to firm back up. If you don't chill the cookies again they will spread!
While the cookies are chilling, preheat your oven to 350°F.
Bake for 10-12 minutes or until the edges are set. Bake on the middle rack of the oven and rotate the pan halfway through baking for the best results. Let the cookies cool on the baking sheet for 2-3 minutes, then remove and let them finish cooling on a cooling rack.
Let the cookies cool completely before decorating.
To make royal icing, whisk together the meringue powder, powdered sugar and 1 teaspoon of water. Slowly drizzle in more water until the icing flows slightly, but still holds its shape.
Transfer the icing to a piping bag fitted with a small round tip and draw on some bones!
Notes
Store cookies in an airtight container at room temperature for up to a week.The meringue powder helps stabilize the royal icing and allows it set quickly, but is totally optional! Especially if you're making cookies that will be eaten the same day or you don't really care if they get smudged. Aka if this is a fun activity for kids, just skip the meringue powder.